This invention relates to production of coal in situ wherein vertical wells are drilled into an underground coal seam, the walls are linked together through the coal to form reaction zones and the coal is produced as gases and liquids. The invention more particularly is directed to methods of accomplishing the linkage channels through the coal.
It is well known in the art how to produce coal in situ, the most common method being to set the coal afire underground, with the fire sustained by continuous injection of an oxidizer. By proper control of the oxidizer, a reducing environment is established in the reaction zone in the coal with the resultant generation of combustible gases. If air is used as the oxidizer, produced combustible gases generally range from about 80 to 200 BTU per standard cubic foot.
In the early experiments with burning coal in situ, shafts were excavated from the surface of the earth to the bottom of the underground coal seam. Channels were then dug through the coal to provide communication with at least two shafts. Workmen ignited the coal face and then evacuated to the surface. The fire was propagated by injecting an oxidizer such as air into one shaft and removing the products of reaction from the second shaft. In this manner a low BTU gas was generated with a heat content in the order of 150 BTU per standard cubic foot. As the burning proceeded and the linkage channel became larger, the heat content of the generated gases would become lower and lower due to oxygen bypass of the burning face. A part of the injected oxidizer would be consumed in the fire and a part would proceed to the exit shaft where the hot low BTU gas would be further burned. In severe cases the resulting flue gases would have a heat content too low for combustion and were therefore useless as a fuel gas.
One of the prime objectives of early experiments in producing coal in situ was to minimize the time workmen were required underground. After many years of experimentation it became apparent that underground workmen would not be required if wells were drilled into the coal seam. This raised the problem of how to link the wells together with a communication passage through the seam. Through the years various linkage schemes were tried including hydraulic fracturing, directional drilling, explosive fracturing, electro-linking using electrical current, various methods of burning the channel and the like.
More experimental work on linkage has been performed in Russia than the combined experimental work done in the other countries of the world. The Russian technicians have perfected a reliable method of linkage using a reverse burn between two or more vertical wells. A detailed description of the successful linking procedure may be found in U.S. Pat. No. 4,036,298 of Kreinin et al. In its elementory form the Russian procedure provides for two wells drilled to the bottom of the coal seam. High pressure air is injected into a first well and hot ignition material is placed into a second well. The air injected into the first well will migrate radially outward and a portion of the air will reach the second well, causing ignition of the coal seam and propagation of the underground fire through the coal seam towards the on coming oxygen supply. The air passing through the coal seam proceeds through paths of least resistance, a path that is unknown to the operator except in the most general sort of way. Thus the channel burned as the fire proceeds from the ignition well to the injector well is always something other than a straight line, and often is a path quite circuitous in nature. As long as the burned channel remains near the bottom of a flat coal seam, straightness of the path is not a critical consideration. Should the burned channel have significant deviations in a vertical direction, difficult operating problems will arise later in the production cycle due to flame override.
Linked vertical wells using the Russian procedures work exceptionally well when there is a thin parting in the coal near the bottom of the seam. In this case the oxidizer release point is established in the coal below the parting and the burned channel is thus restrained from migrating upward. Once the reaction zone is well established from the burned channel, the parting is broken by generated heat and roof fall, and the seam is consumed from the bottom up.
In the Russian procedure the linkage burn proceeds as a reverse burn, that is, the burn moves in an opposite direction from the direction of flow of the oxidizer. Once the channel burns through to the oxidizer injection well, permeability to the flow of gases is greatly increased, injection pressure drops significantly and the burn reverses itself and proceeds as a forward burn away from the injection well. In this manner a reaction zone is established in the coal with an oxidizer injected into one well and the products of reaction withdrawn from a second well.
In and around the reaction zone three significant environments are established. At the fire face the environment is highly oxidizing, down stream away from the fire a shortage of oxygen establishes a reducing environment, and the coal adjacent to the fire is subjected to a pyrolyzing environment. In the oxidizing environment coal is consumed and converted into carbon dioxide, sulfur dioxide and water vapor, gases that have little use except for their sensible heat. At these gases proceed down stream into the reducing environment the carbon dioxide is converted to carbon monoxide and the sulfur dioxide is converted into hydrogen sulfide, with further enrichment by the gases of pyrolysis.
There are obvious limits of effectiveness in the Russian system of linkage. A practical limit is established in maximum well spacing due to the requirement of initially injecting the oxidizer in all directions from the injection well. A distant second well might never receive enough oxygen for ignition. Should the path of least resistance between the wells happen to be a path near the top of the seam, flame override and all of its attendant problems are sure to occur. Also in wet coal seams the path of least resistance to air flow normally will be above the water, a situation that sets the stage for flame override.
When a coal seam is an aquifer of significance, it is necessary to lower the water table in the coal. Percolation of water through the coal is quite slow and lowering the water table in a uniform manner is virtually impossible when using pumps to withdraw the water. By placing pumps in sumps below the coal seam the water table can be lowered to the bottom of the seam in the immediate vicinity of the well bore. Water will remain at an angle of repose away from the well bore, and at a point some distance from the well bore, the localized water table can be several feet above the bottom of the coal seam.
In this case of residual water residing in an uneven water table, the path of least resistance to air flow normally is a path that overrides the water. In attempting linkage between two wells using the reverse burn procedure, the resultant linkage channel will stray considerably from the bottom of the seam.
It is possible to substantially remove the free water in a coal seam using procedures as described in U.S. Pat. No. 2,973,811 of Rogers. The methods of Rogers provide for injecting gas such as air into the aquifer under such pressure as necessary to drive the water out of the area of influence. Such pressures are considerable higher than those used in the Russian procedures of linkage, although a certain amount of water displacement occurs in the Russian procedure.
A reasonable amount of free water remaining in a coal seam is beneficial to the reactions of coal gasification, therefore driving all of the free water out of the coal to be gasified is not desirable. Water reacts with hot coal to form carbon monoxide and hydrogen, two desirable gases with heat contents exceeding 300 BTU per standard cubic feet. Water driven out of a coal seam can be made to return by slacking off on pressure. The rate of return, however, is generally too slow to be of commercial interest. Thus it is preferable to leave most of the water in the seam provided linkage can be accomplished at or near the bottom of the seam.
Another method of linkage that is independent of the water content of coal is described in U.S. Pat. No. 4,062,404 of Pasini et al. A well is drilled some distance away from the intended reaction zone and the well is deviated until the bore encounters the underground coal in a direction substantially parallel to the seam. Directional drilling continues along the bottom of the seam for the desired distance planned for the reaction zone. The circuit is completed by drilling a vertical well to intercept the bottom of the deviated hole. Such an arrangement provides a channel at or near the bottom of the seam, but has the disadvantage of difficult and costly drilling procedures.
Still another method of linkage is described in U.K. Pat. No. 756,852 of Montagnon which provides for establishing a permeable channel with a flow of electric current between two points in the coal seam. The flow of electric current is somewhat analogous to the flow of air, in that the current will flow through the path of least electrical resistance. Coal, being a non-homogeneous rock, has unpredictable paths of electrical circuits. Over long distances between electrodes the likelihood increases for the path to stray substantially above the bottom of the coal, resulting in a path that promotes flame override.
Flame override can be a serious detriment to successful production of coal in situ. The natural tendency of a fire is to burn upward as long as there is a source of fuel in that direction. The worst case in the reverse burn procedure for linkage occurs when the injected air migrates to the top of the seam and persists in that location until it nears the location of the lower pressure in the ignition well. The burned channel, for the most part, will lie at the top of the seam. Upon burn through and the establishment of a reaction zone, the two wells will appear initially to be performing satisfactorily, with produced gases containing approximately 170 BTU per standard cubic foot. The first sign of trouble is signalled by a steady drop in the BTU content of produced gas. The reaction zone, with no fuel above it, is gradually becoming engulfed in its own ashes. A partial remedy can be applied by significantly increasing the velocity of the gases through the reaction zone, thus picking up the ashes into the flue gas for removal above ground. Such a procedure defeats one of the purposes of in situ gasification of coal; that is, to leave the ash content of the coal underground. Increased velocities of the oxidizer also aggrevates the oxygen by pass problem where combustible gases are subjected to unplanned burning underground with the resultant destruction of combustible gases. Also, attempting to burn an underground fire downward is something other than a rewarding task.
From the foregoing it is apparent that successful gasification of coal in situ requires reaction zones that begin at the bottom of the coal seam. In this mode the fire has the preponderence of the fuel supply above it and the ashes fall out of the path of the fire as it seeks new fuel. Also from the foregoing it is apparent that a lengthy reaction zone is desirable because the reducing environment portion of the underground channel provides the setting for generation and recovery of combustible gases. In the Russian procedures for linkage and establishment of reaction zones, well spacing is generally limited to short distances in the order of 70 feet. Greater distances between wells is desirable from an economic point of view as well as the desirability of having a longer distance for a reducing environment in the underground channel. Well spacings greater than that of the Russian procedures would provide more favorable economics and provide a setting for improved performance of the in situ reactions. Such lengthened spacing requires a correspondingly effective linkage procedure.
In U.S. Pat. No. 4,010,801 of the present inventor, methods are taught wherein a blind hole burn in coal creates underground channels and reaction zones for the production of coal in situ. The procedures of the present invention extend the teachings of U.S. Pat. No. 4,010,801 to include methods of linking two or more wells by burning channels along the bottom of the coal seam.